There is a trend amongst commercially available granular detergents towards higher bulk densities and towards granular detergent compositions which have a higher content of detergent active ingredients. Such detergents offer greater convenience to the consumer and at the same time reduce the amount of packaging materials which will, ultimately, be disposed of.
Many of the prior art attempts to move in this direction have met with problems of poor solubility properties arising from low rate of dissolution or the formation of gels. A consequence of this in a typical washing process can be poor dispensing of the product, either from the dispensing drawer of a washing machine, or from a dosing device placed with the laundry inside the machine. This poor dispensing is often caused by gelling of particles, which have high levels of surfactant, upon contact with water. The gel prevents a proportion of the detergent powder from being solubilized in the wash water which reduces the effectiveness of the powder. This is a particular problem at low water pressures and/or at lower washing temperatures.
Further, there has been another recent trend towards reducing or eliminating the use of phosphate builders, which have generally been replaced with zeolite (crystalline aluminosilicate). Detergents containing zeolite builders have been found to be poorer dispensers than detergents containing phosphate builders.
EP-A-0 578 871 describes a process which seeks to make a high bulk density detergent composition which dissolves rapidly and dispenses effectively. The process involves formulating a base powder with a particle size distribution between 150 microns and 1700 microns in combination with additional filler ingredients whereby at least 20% by weight of the filler particles is less than 150 microns. The filler particles include salts of citrate, sulphate, (bi-)carbonate and silicates.
WO95/14767 relates to the poor dispensing of high density, non-spray-dried detergent powders, and discloses the use of a citric acid salt which has a Rosin Rammler particle size of less than 800 microns.
WO94/28098 discloses a non-spray-dried detergent powder comprising a combination of an ethoxylated primary C8-18 alcohol, an alkali metal aluminosilicate builder and 5 to 40 wt % of a water-soluble salt of a citric acid.
EP-A-0 639 637 discloses the replacement of perborate bleach with an alkali metal percarbonate to improve the dispensing profile and dissolution rate of a detergent. Citrate or mixtures of citrate with sulphate or carbonate can be used to coat the percarbonate bleach. EP-A-0 639 639 contains a similar disclosure in this respect.
Other ways to improve dispensing include the use of an effervescence system. If the detergent contains an effervescence system then the generation of a gas such as carbon dioxide pushes the particles of the detergent apart, and prevents them from gelling.
The use of effervescence to improve the dispersibility of granular materials has been used extensively in pharmaceutical preparations. The most widely used effervescent system in this respect is citric acid in combination with bicarbonate. The use of this simple effervescent system has also been described for improving the dispersibility of pesticidal compositions for controlling water-borne pests, e.g. GB-A-2,184,946.
U.S. Pat. No. 4,414,130 discloses the use of a readily disintegratable builder particle with a zeolite-based detergent. It also discloses the use of an effervescence material to improve the dissolving and dissolution of the particles. Sodium carbonate or sodium bicarbonate may be combined with the zeolite binder mix and the balance of the detergent may include citric acid, monosodium phosphate, boric acid or other suitable acidifying material, preferably encapsulated or agglomerated with bicarbonate, for reaction with it to generate carbon dioxide.
WO92/18596 discloses that improved solubility/dispersion for granular detergents can be achieved by admixing sodium carbonate and citric acid in a specified weight ratio of from2:1 to 15:1.
EP-A-0 534 525 discloses the use of citric acid with a specified particle size range of 350 to 1500 microns.
The addition of citric acid results in a reduction in alkalinity. Such an alkaline pH promotes cleaning, stain removal and soil suspension, there is need to minimize the level of citric acid used. Also citric acid is a relatively expensive ingredient which further reinforces the need to keep the level of citric acid very low. We have also surprisingly found that the present invention allows low levels of the acid to be satisfactorily used in the detergent composition.
All documents cited in the present description are, in relevant part, incorporated herein by reference.